If a body is in freefall and has reached its terminal velocity say 56 metres/s, its acceleration is zero, how do you work out with what force it would hit the ground

Should be just mass of falling object times speed with which it impacts. You should probably, for the sake of simplicity, assume that the landing area has no elastic properties. The force would be the same even if the thing lands in a bowl of Jell-O, but it would be distributed a lot differently.

If you have the initial speed of impact, you will need the mass of the object PLUS either the distance that the object went onto the ground during impact, or the time interval of the impact with the ground.

IF you have the time of the impact, F = mv/t , where v is the "change in velocity" (final velocity is zero). This is using the impuolse-momentum theorem.

If you are given d, the distance of impact (depth of crater), F = (mv^2)/2d
This is using kinematics and dynamics

I cannot believe that I confused 'force' and 'energy'. Sorry, guys. The terms just kind of swapped places in my head and I didn't notice. It's like when I'm stuck on a crossword puzzle answer for hours, then suddenly realize that I've misread the clue eight times in a row. (In my defense, I was on my 7th beer at the time.)